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<br />C,J <br />,...-l <br />C; <br />...-l <br /> <br />,- from the river in the LIpper portion of the Gl'and Ve1ley area. Insome <br />.-.~ places the aquifer is under artesian pressure, and the sal inity of the ~Iater <br />in the aquifer varies from 6,000 to 36,000 parts per million of total dis- <br />solved salts. Salinity concentrations normally increase with depth. <br /> <br />The topography in the irrigated area is gently slopinq to rol1ir:g. The <br />major overall slope is to the river. Most of the irrigated soil is in the <br />o to 2 percent and 2 to 5 percent slope groups. Some breaks and hummocky <br />soils are also irrigated. <br /> <br />Soils adjacent to and above the floodplain area, outside the valley refill, <br />are residual soils developed on the Mancos Shale, and there is no distinct <br />aquifer. There are scattered lenses of sand which may serve as local aqui- <br />fers and provide some local subsurface drainage. An analysis of the soil <br />properties in the irrigated area shows the permeability to be distributed <br />as follows: 64 percent - 0.2 to 0.3 inches per hour; 28 percent - 0.4 to <br />.0.5 inches per hour; and 8 percent - 1.0 to 2.0 inches per ho~r. <br /> <br />Nature of Sa1 initv Problems <br />- <br /> <br />Return flows from the irrigated area to the Colorado River probably contain <br />as much salt now as they did at the inception of irrigation. It can be <br />assumed this level will continue unless excess water which dissolves salt <br />from the Mancos Shale are reduced and controlled. Some of the major washes <br />travers i nq the i rri qated area and d is chargi ng into the ri ver have flows <br />with salinity levels in excess of 6,000 parts per million of total dissolved <br />solids. Estimates of the amount and salt concentration of return flows have <br />been made in this report; however. more rel iable data and analysis of the <br />surface and groundwater return systems are needed before impacts of alter- <br />nati ve control prog'tams can be adequately made. <br /> <br />The water distribution system is complicated as it involves five major <br />ditch companies and about 1,200 1 andowners with constant f1 O~I water rights. <br />Figure 1 shows the Grand Valley canal distribution system. Most of the <br />main and lateral canals are earth lined but some limited sections have been <br />concrete or bituminous lined in recent years. While accurate figures on <br />canal seepaqe are not available, one study estimates that main and lateral <br />canal seepage accounts for 30 percent of the salt contribution from the <br />irrigated area. Others have expressed opinions that this is a low figure. <br />The total contribution of salts from the Grand Valley area (including up- <br />land drainage) can be determined by comparing data from U. S. Geological <br />Sur;ey qages as shown in Tab1,~ '. <br /> <br />The water disposal systems include 14 natural wastes, 15 to 30 feet deep, <br />additional man-made community or group drains and many miles of on-farm <br />collection and drainage ditches. Most of the return flow finds its way <br />into the natural washes befor,: returning to the Colorado River. <br /> <br />- 2 - <br />